This invention relates generally to the art of microstrip antennas, and more particularly concerns microstrip antennas having a plurality of stacked microstrip elements.
In general, a microstrip antenna, which is a radiating element, comprises a ground plane of electrically conducting material, an intermediate layer of dielectric material, and an upper conducting plane. The conducting plane is electrically fed either from the side, or from below through the ground plane and the dielectric layer. Such a microstrip antenna is excited like a cavity, with a radiating electric field being established between the respective edges of the ground plane and the conducting plane.
Microstrip antennas have several significant advantages which make them particularly suitable for use in airborne, satellite, and similar applications. Other types of antennas have better electrical properties than the microstrip antenna; however, a microstrip antenna is very light in weight, does not require much space, and being formable, can be integrated into other structures, such as the wing or body of an airplane.
However, the use of microstrip antennas in airborne applications has heretofore been restricted, due to the inherent narrow bandwidth of the microstrip antenna. Most systems which interface in a signal sense with antennas, such as a radar, usually operate over a bandwidth of approximately 10%, i.e. the antenna operates over a range of frequencies .DELTA.F which is 10% of the center or resonant frequency of the antenna.
To interface effectively with other systems, the antenna should have a comparable bandwidth. Microstrip antennas, however, have a bandwidth of between 1-2%. In some applications, a narrow bandwidth antenna can be tolerated, and hence there has been some use of the microstrip antenna, although its narrow bandwidth has heretofore prevented its wide spread use in applications, i.e. airborne, for which it is otherwise ideally suited.
Various techniques have been used to improve the bandwidth of microstrip antennas. In one technique, a slot is cut in the conducting surface element, while in another technique, the thickness of the dielectric substrate is increased or the dielectric constant of the substrate is varied. Many of these techniques, however, have proven, for one reason or another, to be either impractical or to cause a serious degradation in antenna performance. Increasing the thickness of the dielectric substeate has been found to have the most profound effect on bandwidth, but increasing the substrate thickness defeats one of the primary advantages of the microstrip antenna, i.e. its small dimensions.
Accordingly, it is a general object of the present invention to provide a microstrip antenna which solves one or more problems of the prior art noted above.
It is another object of the present invention to provide such an antenna which has a bandwidth substantially greater than that previously obtained with microstrip antennas.
It is a further object of the present invention to provide such an increased bandwidth antenna which retains all of the other advantages of the conventional microstrip antenna.
It is an additional object of the present invention to provide such an increased bandwidth antenna without increasing substantially either the size or weight of the antenna.